The present invention relates in general to the field of an electrical isolation layers, and more particularly, to an electrical isolation layer arranged between a first conductive material and a second conductive material. The isolation layer is capable of inhibiting, if not preventing, either conductive material from electrically passing or bleeding into the other conductive material and thereby causing a high resistance connection among the conductive materials. The present invention has particular application when arranged between a conductive roebel filler and conductive strands sheathed by a porous insulator carrying a high voltage within a stator of a dynamoelectric machine used in a power generation plant.
Many power generation plants produce electricity by converting energy (e.g. fossil fuel, nuclear fission, hydraulic head and geothermal heat) into mechanical energy (e.g. rotation of a turbine shaft), and then converting the mechanical energy into electrical energy (e.g. by the principles of electromagnetic induction). Some of these power generation plants, such as a fossil-fuel power generation plant, comprise a turbine, a generator and an exciter.
One aspect of the above-described power generation scheme involves conductive copper strands located within axially extending slots of the generator""s laminated stator core. As a rotor rotates within the annular stator, magnetic flux induces an electric current through the conductive strands. To compensate for the decrease in magnetic flux density along the transverse (Zxe2x80x94Z) depth of the conductive strands, the conductive strands are transposed along their axial (Yxe2x80x94Y) length such that each strand experiences a similar amount of flux and thus carries a similar amount of current. The most common type of transposition is disclosed in U.S. Pat. No. 1,144,252 to Roebel and known in the art as roebelling, although other transposition types are known such as that disclosed in U.S. Pat. No. 4,128,779 to Salon.
Each conductive strand is typically sheathed with an inexpensive insulation, such as DACRON fiber glass (which has a relatively open weave with small porous micro-openings), to insulate the individual strands from each other. The insulated strands are then roebelled. A filler is pressed into the roebel offsets (i.e. open space between the robelled insulated strands). The filler is advantageously conductive, such as resin rich felt or mica, to inhibit it from undergoing partial electrical discharge activity and to meet the power factor tip-up requirements in order to maintain a high resistance between strands. Since the conductive filler can electrically pass or bleed through the openings in the DACRON fiberfill glass strand insulation to the conductive strands (and/or vice-versa), low resistance electrical connections among the strands and/or filler can exist.
These low resistance connections are undesirable for a variety of reasons, such as compromising group-to-group coil electrical testing and meeting the power factor tip-up requirements in order to maintain a high resistance between strands.
There is thus a need to reduce, if not eliminate, low resistance connections among the strands and/or filler. There is also a need to electrically isolate a first conductive material from a second conductive material. There is also a need for a strand assembly that improves upon the prior art.
The present invention reduces, if not eliminates, low resistance connections among the conductive strands and/or conductive filler, as well as electrically isolates a first conductive material from a second conductive material.
One aspect of the present invention thus involves an electrical isolation layer system comprising, a first conductive material comprising a plurality of copper strands; a second conductive material comprising a roebel filler; and a NOMEX fiber spun laced felt having a dielectric strength of at least 300 volts per millimeter interposed at least partially between the copper strands and the roebel filler.
Another aspect of the present invention thus involves a strand assembly for use within a stator of a dynamoelectric machine of a power generation plant, comprising a plurality of roebelled conductive strands that extend along a generator length; an insulator sheathing each of the strands; a conductive filler at least partially surrounding the insulated strands; and an electrical isolation layer disposed at least partially between the insulated strands and the conductive filler material.
A method of forming a strand assembly that extends along an axial length, comprising sheathing a plurality of conductive strands with an insulating material; roebelling the insulated strands; arranging an electrical isolation layer at least partially over the insulated strands; and arranging a conductive filler at least partially over the insulated strands whereby the isolation layer electrically isolates the strands from the filler.
Further aspects, features and advantages of the present invention will become apparent from the drawings and detailed description of the preferred embodiment that follows.